The main goal during the exploitation phase of an oil reservoir, from a technical-economic point of view, is to obtain the optimal hydrocarbons recovery, so that it remains the least amount of residual oil in the reservoir. In order to increase the amount of oil, it is used the secondary and/or enhanced recovery processes, which mainly consist in injecting fluid for providing additional energy to the reservoir, taking advantage of this energy in the displacement of hydrocarbons towards production wells.
The tracer tests among wells are a widely used tool in the recovery processes, in order to determine the flow trajectories of injection fluids, as well as to detect high permeability zones or drainages that cause a disproportionate distribution of injected fluids, which can be reflected on an efficiency process reduction.
In documents found in tracer tests literature, the sampling test is performed through a visit to the field of selected production wells, by trained technical staff, this task is carried out according to a previously established sampling test program during the design stage of the activities for the tracers injection to the reservoir. This program usually takes into account a high sampling test frequency the days immediately after the tracer injection, so that going down its frequency as long as the time passes through. The reason of high frequency at the beginning is the possibility of the presence of tracer due to drainage which breakthrough the tracer in the production well very quickly. This matter produces a very short tracer response but at the same time of great magnitude so that it can be only possible to reconstituted if it is possible to have a sufficient number of sampling tests. Otherwise, when there is not drainages, the tracer flow more slowly in the porous media, so the scheduling of sampling tests collection is at least one year, and then to accomplish that the tracer response more closely reflect what happens in the reservoir. Taking into account the above description, the cost of the sampling test of a tracers test rises sharply, due to the large amount of sampling tests. It is worth to say that a substantive part of the cost of a tracer project corresponds to the analysis of sampling tests, and often it is sacrificed the number of sampling tests in order to reduce the project costs. However, the information obtained from tracer tests is directly proportional to the number of analyzed sampling tests.
One of the main problems that arise when interpreting a tracers test results, and even, reaching some failure cases, is caused by a poor and/or insufficient monitoring program. This may be due to several factors, mainly to an inadequate program design of the sampling test, or it could also be due to other causes, such as, difficulty of moving through long distances for carrying out the sampling tests, impossibility to perform the sampling tests due to affectations caused by farmers who did not allow access to the wells, remote offshore platforms, or it could also be due to the lack of available resources (human, economics) for sampling tests:
The main advantage that represents the radioactive tracers is the possibility of working with small volumes for its injection and in many cases, especially for gamma emitters, its facility for being detected in-situ. However, the radiation measurements for radioactive isotopes of low energy beta emitters such as tritium (18 keV maximum beta energy) and carbon-14 (155 KeV) are not carried out in the field, because their analysis is carried out with special low level count equipment, therefore all samples are sent for their analysis to specialized laboratories that have liquid scintillation counters equipments. In another case, the use of radioactive isotope tracers that emit gamma radiation, such as: 57Co, 58Co and 60 Co, 192Ir or 131I, they make much easier their detection, which can be achieved through scintillation crystals. The sodium iodide detectors activated with thallium, NaI (TI), are widely used for the detection of gamma radiation, which given its characteristics make possible that they can be used in the field, which allows it become unnecessary to perform a sampling test and then to send it to the laboratory for radio-chemical analysis.
Currently, for measuring gamma radiation, there are a several commercial laptops, however, its use is focused on general applications, among these kind of commercial mobile computers it can be mentioned the following models: 1000 Inspector, Inspector 2000 Canberra brand, and others from the Ortec brand.
It is important to mention that if commercial equipment are intended to be used for detecting radioactive tracers in a intrusive way, which is known by the
term of “on-line detection,” in the head of production wells, these equipments would present serious disadvantages in compared with the system developed in this invention, such as:
    1. Non-intrusive measuring drill. They cannot directly measure the radiation contained in fluid from of the reservoir.    2. They are portable, but with battery life which last from 3 to 10 hrs. which does not allow us to connect them to the wells permanently.    3. They do not have data storage capacity for testing lasting long periods of time (months).    4. Temperature operation is very limited (maximum 55° C.).    5. They do not operate on a autonomous manner, i.e. they require the permanent presence of an operator.
It is worth to say that it have been reported (Zemel, 1995) applications in tracer tests, where it is mentioned that radioactive tracers measurements can be performed in-situ by using detectors NaI (TI), however, features of the measurement system are not specified and even less if they are commercially available equipment, or having similar characteristics to the system of the present invention.
There are also commercial tools or systems (Spectral Gamma Ray Tool, TracerScan, etc.) from different companies like Halliburton, Schlumberger, International Protechnics, etc., whose application is the natural gamma radiation log test, or also gamma spectroscopy applications, within oil wells, these tools are used to characterize the stratums, and they operate at conditions of high temperature and pressure. However, these tools are designed to operate inside the wells, so they do not meet all the features and operating purposes of the measurement system that is result of this invention.
Likewise, with regarding to the above they are published large number of patents relating to tools and systems to make profiles of gamma radiation inside the wells, focusing to different applications. For example, in U.S. Pat. No. 4,007,366, relate equally to systems and apparatus for take a radiation intensity profile of tracer in different runs that are performed inside the wells. The arrangements consists of a background tool (drill), which has two types of radiation detectors Geiger Müller, a device for injecting a tracer charge inside the well, a telemetry module to transmit data between the drill and the surface equipment. The pulses generated by detectors, are sent to the surface equipment through a cable record. The unit on the surface, has all the postcards for the management of the pulses from the background tool, electronic arrangements for corrections of the readings, discrimination circuits, counters, power supplies to provide the energy needed to power electronic circuits in the equipment fund, etc. Given the characteristics of the detectors used, this system can not differentiate between two or more tracers used, nor can operate autonomously.
Other patents mentioned techniques developed for specific applications related tools also for operate in the interior of the wells, such is the case of U.S. Pat. No. 4,481,597, which refers to an analog to digital converter or spectrum analyzer for use in a drill for making logs of spectrum gamma ray within the wells. The system converts analogics pulses generated by the photomultiplier tube in a digital representation or digital word. This digital representation has the form of numbers representing the energy of gamma rays or other types of nuclear radiation that produces scintillations in the crystal detector, which is optimally coupled to the photomultiplier tube. The digitized value is transmitted by the drill to the surface through cable record.
Also there are published other developments related to the sampling of fluids in wells, as mentioned in the reference U.S. Pat. No. 4,454,772, which describes a new method for automated fluid sampling wells. This method is basically of a series of solenoid valves to inject fluid from the well to a number of sample containers are filled one after another, through the valves that are electrically driven by a programmable switch. Later, with the series of containers collected samples are sent for laboratory analysis. The novelty of the method of the present invention is to automate the sampling of fluid from the wells, thus avoiding moving staff to the sampling points.
The references mentioned above were created for entirely different applications of the present invention, by virtue of this we have implemented an online measurement system of radioactive tracers in the wellhead in an offshore producer of oil, which allows continuously monitor the presence or not presence of three different tracers, above to determine with greater precision the times of arrival of the tracer, while eliminating the need to allocate staff to carry out sampling operations, with all advantages that this represents.
Therefore, one of the objects and advantages of the present invention is to provide a measurement system that allows online monitoring and permanent values of tracer concentration, and is able to operate autonomously according to a program monitoring previously established based on the design and objectives of the injection of tracer to the site, and thus have more data from the tracer activity, which reproduce the response curves of tracer, which contribute to reduce the level of uncertainty and increase efficiency in the analysis and interpretation of results.